I have no direct experience with Trieste. But I have never had trouble getting a reliable taxi at any Italian train station, except Naples (and I've used them in Florence, Rome, Bologna, and Milan, among others). You just ignore anyone who approaches you with "taxi?" within the station, until you get to the official taxi stand (always well labeled). Actually, I've only had these offers for non-official taxis in Rome. If trains are arriving, people will want cabs, so a 9:30 PM arrival shouldn't be a problem at all.